A conventional combustion engine for a motor vehicle, such as for instance a car, lorry, towing vehicle or bus, is provided with injectors for injecting desired quantities of fuel at desired moments into the cylinders of the engine. The injectors are controlled by means of an engine control unit, which controls the on-times for each individual injector so as to thereby control the quantity of fuel to be injected into the cylinder associated with the injector. It is a common practice to establish suitable on-times for the injectors with the aid of an on-time lookup table comprising preset on-time values for the injectors linked to fuel injection quantity values, where the fuel injection quantity values represent different desired quantities of fuel to be injected into a cylinder of the engine and the on-time values represent the associated on-times for an injector. For a common rail fuel injection system, the on-times of such a lookup table are linked both to such fuel injection quantity values and to fuel pressure values representing different fuel pressures in the fuel accumulator of the common rail fuel injection system. Alternatively, the on-times for the injectors may be established with the aid of an on-time calculation model based on such fuel injection quantity values and fuel pressure values. The engine control unit determines the present fuel injection quantity value, i.e. the desired quantity of fuel to be injected by an injector at the present cylinder stroke cycle, with the aid of a regulator in dependence on the present operating conditions of the vehicle. Based on the present fuel injection quantity value and the present fuel pressure value, the engine control unit will establish a present on-time for an injector with the aid of said on-time calculation model or lookup table. This on-time is expected to result in the injection of the desired fuel quantity, i.e. a fuel quantity corresponding to said present fuel injection quantity value. However, in course of time, the characteristics of an injector may change in such a manner that the on-time values established with the aid of the on-time calculation model or lookup table will not give the expected fuel injection quantities. Such deviations between expected and actual fuel injection quantities are undesired for several reasons.
It is known, e.g. from U.S. Pat. No. 4,421,088, that the quantity of fuel injected into a cylinder by an injector connected to a fuel accumulator of a common rail fuel injection system is related to the pressure drop of the fuel in the fuel accumulator during the injection. U.S. Pat. No. 5,176,122 discloses a manner of establishing the quantity of fuel injected into a cylinder by an injector connected to a fuel accumulator of a common rail fuel injection system based on a measurement of the pressure drop of the fuel in the fuel accumulator during the injection. According to U.S. Pat. No. 5,176,122, the thus established fuel injection quantity is used for correcting the quantity of fuel to be injected by the injector.
However, there are fluctuations in the pressure of the fuel in the fuel accumulator of a common rail fuel injection system. The amplitudes of these fluctuations are rather large in relation to the normal magnitude of the pressure drop of the fuel in the fuel accumulator during a fuel injection, which may result in an inaccurate value of the fuel injection quantity established in the above-indicated manner based on a measurement of the pressure drop of the fuel in the fuel accumulator during a fuel injection.